a) Field of the Invention
The present invention is directed to a reflective imaging system for an x-ray microscope for examining an object in an object plane, wherein the object is illuminated by rays of a wavelength of less than 100 nm, particularly less than 30 nm, and is imaged in a magnified manner in an image plane.
b) Description of the Related Art
Microscopic examination of objects by x-ray radiation is becoming increasingly important especially in the semiconductor industry. Smaller structure sizes consistently require increasingly higher resolutions which can only be achieved by shortening the examination wavelength. This is particularly important in microscopic inspection of masks for the lithography process. Lithography with extreme ultraviolet (EUV) radiation represents the most promising solution for chip fabrication in the coming years.
Numerous different technical solutions for x-ray microscopes are known in the prior art.
U.S. Pat. Nos. 5,222,113; 5,311,565; 5,177,774 and EP 0 459 833 show x-ray microscopes in which zone plates are provided in the projection optics for imaging. These Fresnel zone plates are wave-optic imaging elements in which the light is diffracted at a system of concentrically arranged circular rings. The disadvantage in using Fresnel zone plates in the imaging systems with a plurality of optical elements in the area of the x-ray radiation is that Fresnel zone plates are transmissive component parts which result in large light losses because of the poor transmission in the x-ray range.
U.S. Pat. Nos. 5,144,497; 5,291,339 and 5,131,023 concern x-ray microscopes in which Schwarzschild systems are used as imaging systems. In these x-ray microscopes, the beam paths are laid out telecentric to the object under examination, which makes reflective imaging of objects difficult.
Another disadvantage in systems of the type mentioned above for use in examination of objects, particularly those used in the field of x-ray lithography, is their large structural length for achieving a sufficient imaging scale. This makes it more difficult to use them, for example, in inspection systems for examining masks in EUV projection exposure installations.
U.S. Pat. Nos. 6,469,827 and 5,022,064 disclose the use of diffractive elements for spectral selection through diffraction of x-ray radiation. In both of these references, however, these elements are only used for spectral separation and selection of x-ray radiation and not for correcting or improving imaging characteristics. This system is also laid out telecentric to the object, which impedes reflective imaging of objects.
The use of a diffractive optical element with a refraction-reinforcing and achromatizing effect for an objective, particularly a microscope objective, is described in DE-OS 101 30 212. However, an objective of this kind can not be used for EUV radiation because of the transmissive optical elements. Since EUV radiation, in contrast to UV radiation, is absorbed to a very great extent in virtually all materials, the use of optical components relying on transmission is not possible.
A reflective x-ray microscope for examining an object for microlithography in an object plane with radiation having a wavelength of less than 100 nm, particularly less than 30 nm, is known from JP 2001116900. The x-ray microscope disclosed in this application is a Schwarzschild system with a concave first mirror and a convex second mirror. In contrast to the systems described above, the beam path for examining the object is not telecentric to the object, so that reflective examination, e.g., of EUV reflection masks, is possible. A disadvantage in this system consists in the very large structural length for achieving large imaging scales.
Another x-ray microscope arrangement is described, for example, in the applications DE 102 20 815 and DE 102 20 816. In these applications, the imaging optics are designed as a purely reflective system and optimized with respect to small structural length with high magnifications. This is achieved through the use of highly aspherical mirrors, among other things. A disadvantage in these arrangements is that the manufacturing tolerances for the aspherical mirrors are extremely exacting in order to achieve high image quality and demanding requirements must therefore be imposed on the manufacturing technology and measuring technique.
The object of the present invention is to develop an imaging system for an x-ray microscope which avoids the disadvantages known in the prior art. Further, a high imaging quality is achieved at a reasonable manufacturing cost.
According to the invention, this object is met by the characterizing features of the independent claims. Preferred further developments and constructions are the subject of the dependent claims.
The proposed imaging system contains all of the optical elements associated with imaging optics and generates a corresponding intermediate image by means of extreme ultraviolet (EUV) radiation. This can be further processed, i.e., further magnified, by additional imaging systems.
The imaging system according to the invention can be used, for example, in photolithography through the use of EUV radiation of 13.5 nm.
The invention will be described in the following with reference to an embodiment example.